A microfluidic-based microwave interferometric inductance sensor capable of detecting single micron-size superparamagnetic particles in flow
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A microfluidic-based inductance sensor operating at 1.5 GHz is presented that can detect single 4.5 μm superparamagnetic particles flowing in a microfluidic channel. The particles are detected as they pass over a micron-sized planar gold loop electrode, with a maximum signal-to-noise ratio of 26.3 dB for an 80 μm/s flow rate; the magnetic beads are simultaneously observed with microscope images. The sensor consists of a coupled-line resonator and microwave interferometric system coupled to the loop electrode that is integrated within a polydimethylsiloxane-on-glass microfluidic chip assembly. A time-averaged inductance change caused by a single particle is related to the real part of its magnetic Clausius-Mossotti factor. The effective real part of the magnetic permeability for a particular particle is estimated to be 1.13 at 1.5 GHz. The sensor detects magnetic particles in flow and does not require an external biasing magnetic field, which distinguishes it from other magnetic microparticle sensors.